In normal communication systems, antenna bandwidth (BWant) is much larger than the bandwidth of the signal (BWsig) to be transmitted and received to make sure that all the signal power is received and not corrupted in any way.
Some antennas become more efficient as their bandwidth is reduced. In particular that is true for resonated magnetic link antennas as the ones typically used in connection with hearing aids. However, if the antenna bandwidth is too narrow, the antenna frequency response will distort the communication signal. The trade off between antenna efficiency and signal distortion leads to a certain optimal antenna bandwidth of existing systems.
EP 0 261 935 A2 discloses an electrically small antenna for receiving or transmitting radio signals. EP 0 261 935 A2 describes the tuning of the centre frequency to a specific value thereby eliminating the effect of tolerances of reactive components. This provides for operating the antenna with a smaller natural bandwidth (and thereby a better antenna efficiency).
US 2008/158076 A1 describes a narrow band antenna for use in wide band (multi-channel) systems. The effective bandwidth of a dynamically adjustable antenna with a narrow natural bandwidth delineated by a first frequency change can be moved from the natural bandwidth to another narrow bandwidth of interest within a wide band spectrum using a tuning circuit. The natural (narrow) bandwidth is still wider than the modulation bandwidth.